Linux Name Services

Chapter 4: Linux Name Services

One of the most fundamental services on a TCP/IP network is

translates hostnames into IP addresses. In Chapter 3, "Login Services," we configured telnet and

ftp. Without name service, a user connecting to crow enters

name service. It is the service that

telnet 172.16.5.5

With name service, that same command is

telnet crow

The result is the same. In either case, the user connects to the host at address 172.16.5.5. But most

users prefer hostnames because they are easier to remember and easier to use. This is particularly

true in the global Internet. It is possible to guess that http://www.sybex.com/ is a valid name, but

there is no intuitive way to guess the address 206.100.29.83.

Linux systems use two techniques to convert hostnames to addresses: the host table and the

Domain Name System (DNS). The /etc/hosts file is a table that maps names to addresses. It is a

simple text file that is searched sequentially to match hostnames to IP addresses. The Domain

Name System is a hierarchical, distributed database system with thousands of servers across the

Internet, handling name and address queries. DNS is far more important than the host table for the

operation of the Internet, but both services play a role. This chapter's discussion of name services

begins with a quick look at the host table.

The

hosts File

Each entry in the /etc/hosts file contains an IP address and the names associated with that address.

For example, the host table on crow might contain the following entries:

Listing 4.1: A Sample Host Table

$ cat /etc/hosts

127.0.0.1 localhost localhost.localdomain

172.16.5.5 crow.foobirds.org crow

172.16.5.1 wren.foobirds.org wren

172.16.5.2 robin.foobirds.org robin

172.16.5.4 hawk.foobirds.org hawk

The first entry in this table assigns the name localhost to the address 127.0.0.1. (Every computer

running TCP/IP assigns the loopback address to the hostname localhost.) Network 127 is a special

network address reserved for the loopback network, and host 127.0.0.1 is the loopback address

reserved for the local host. The

computer to address itself in exactly the same way that it addresses remote computers. This

simplifies software because the same code can be used to address any system, and because the

address is assigned to a software loopback interface (lo), no traffic is sent to the physical network.loopback address is a special convention that permits the local